This invention relates to a modified vegetable protein adhesive material useful as a binder and pigment structuring additive for paper coatings as well as a process for producing the same.
Vegetable protein materials are well known as adhesive binders for pigment containing coatings which are used for the coating of paper. Pigment containing coatings provide the paper with a desirable finish, gloss, and smoothness. The functions of the pigment in the coating are to fill in the irregularities of the paper surface and to produce an even and uniformly absorbent surface for printing. The adhesive binder functions to bind the pigment particles to each other as well as to the surface of the coating, as well as interact with pigment particles to structure the pigmented coating and produce the desired porosity and surface smoothness. The selection of a suitable adhesive or binder is therefore an important factor in the quality of a coating applied to paper.
Vegetable protein materials have been extensively used as adhesive binders for paper coatings, and among those materials have been the vegetable protein isolates, typically soy isolate. Isolated soy protein is produced by the treatment of oil free soy bean flakes with an alkaline solution to dissolve the protein which is removed from the non soluble materials by filtration or centrifugation. The protein is then recovered from the solution by the addition of acid in order to precipitate the protein at its isoelectric point. The precipitated protein is then in an unhydrolyzed or generally unmodified state and can be dried and subsequently dispersed in an alkaline medium to form an adhesive binder for use in the preparation of pigment containing paper coating compositions. The unhydrolyzed or unmodified soy protein isolates are generally less desirable as adhesive binders for paper coatings because of relatively low solubility in weak alkaline solutions, high solution viscosity and sensitivity to heat, besides a tendency to form gels at temperatures of about 160.degree. F. Because of these disadvantages, unmodified or nonhydrolyzed soy protein is usually modified in some way to lower the viscosity of coating compositions containing these materials as binders and furthermore to increase the adhesive strength of the protein when it is dispersed in weak alkaline solutions. Modification or hydrolysis of the protein also reduces the sensitivity of the protein to heat. Modification of the protein usually consists of hydrolysis or treatment of the extracted protein in an aqueous dispersion with various alkaline reagents under controlled conditions of pH, temperature, and time; native or unmodified. These conditions dissociate the protein structure into smaller sub-units thereby improving the solubility of this material as a binder in paper coatings.
Another means of modifying the protein is to treat the unmodified protein with various chemical reagents in order to alter the rheological properties of coatings containing the modified protein material as a binder. Generally, the purpose of modifying the protein is to provide for increased fluidity of the protein material in alkaline dispersions and also much lower viscosity of coating compositions prepared with the modified protein material as a binder.
U.S. Pat. No. 2,862,918 describes such a modified protein wherein an isolated soy protein which has been hydrolyzed beyond the gel stage is acrylated by treatment of the hydrolyzed soy protein material with a carboxylic acid anhydride, such as acetic anhydride. U.S. Pat. No. 2,932,589 describes a paper coating composition and process in which a modified soy protein material is produced by reaction of an isolate with a dicarboxylic acid anhydride, such as phthalic anhydride.
U.S. Pat. No. 4,474,694 also describes a modified vegetable protein adhesive binder and a process for producing the same in which an alkaline dispersion of a vegetable protein material is reacted initially with a reducing agent, followed by reaction with a carboxylic acid anhydride to modify the protein material and provide a coating with improved strength and ink receptivity.
In spite of the various improvements that have been obtained by chemical modification of vegetable protein material and the use of these materials in the preparation of paper coating compositions, it would still be desirable to obtain a modified protein material that interacts with pigment particles, changes the rheological properties of the coating and improves the pigment structure in the coating. This objective has been generally achieved in the present invention by the production of a modified vegetable protein material which is suitable in paper coating compositions containing pigments.
It is therefore an object of the present invention to provide a modified proteinaceous adhesive binder of good rheological properties.
It is a further object to provide a modified vegetable protein adhesive binder having good pigment binding and paper coating characteristics.